A team of researchers led by Liming Dai at the University of Dayton, Ohio, has found that arrays of vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) can act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, insensitivity to CO poisoning and tolerance to crossover effect than platinum for oxygen reduction in alkaline fuel cells.
The ability to replace costly platinum with a much lower-cost carbon-based catalyst could lead to more efficient fuel cells that can be affordably mass-produced. A paper on the findings was published in the 6 Feb issue of the journal Science.
Traditionally, fuel cells employ expensive platinum-based electrocatalysts, which cost about $4,000 for a passenger car. The goal is to reduce the major cost of a fuel cell in order to compete with current market technologies, including gasoline engines. Our finding is a major breakthrough toward commercialization of fuel cell technology for various applications.—Liming Dai
The oxygen reduction reaction (ORR) at the fuel cell cathode, which breaks apart the O2 molecule, is enabled by the ORR electrocatalyst material and is a key determinant in the performance of the fuel cell. Platinum has been the conventional choice since fuel cells were developed for the Apollo mission in the 1960s, the authors note, but large scale commercial application has been hampered by the high cost of the metal, its susceptibility to time-dependent drift, and to deactivation by CO. Consequently, a great deal of research has gone into reducing the amount of Pt used, or replacing it with a variety of different materials.
The Dai group found that their VA-NCNTs could catalyze a four-electron ORR process with a much higher electrocatalytic, lower overpotential (the difference between thermodynamic and formal potentials), smaller crossover effect, and better long-term operation stability than that of commercially available or similar platinum-based electrodes in alkaline electrolytes.
In air-saturated 0.1 molar potassium hydroxide, we observed a steady-state output potential of –80 millivolts and a current density of 4.1 milliamps per square centimeter at –0.22 volts, compared with –85 millivolts and 1.1 milliamps per square centimeter at –0.20 volts for a platinum-carbon electrode. The incorporation of electron-accepting nitrogen atoms in the conjugated nanotube carbon plane appears to impart a relatively high positive charge density on adjacent carbon atoms. This effect, coupled with aligning the NCNTs, provides a four-electron pathway for the ORR on VA-NCNTs with a superb performance.—Gong et al. (2009)
The role of nitrogen-doping as shown in this study could be applied to the design and development of various other metal-free efficient ORR catalysts, and these nitrogen-containing carbon nanotube electrodes are clearly of practical importance, the authors note.
Michael Durstock in the Air Force Research Laboratory’s Materials and Manufacturing Directorate; Zhenhai Xia in the University of Akron department of mechanical engineering; and Kuanping Gong and Feng Du in the University of Dayton departments of chemical and materials engineering contributed to the report.
Kuanping Gong et al. (2009) Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science Vol. 323. no. 5915, p. 753 doi: 10.1126/science.1166510